The present invention relates to a backlight device for illuminating a panel display body, such as a liquid crystal display panel, from its back.
Conventionally, for example, as a backlight device for illuminating a liquid crystal display panel from the back, there is often adopted a backlight device called a side light system, which includes a light guide body along the back of the liquid crystal display panel, and a light emitting element arranged at the side of the light guide body, such as a light emitting diode or a fluorescent lamp, and in which light of the light emitting element is made plane emission through the light guide body. This kind of backlight device is very thin and is suitable for brightly illuminating the liquid crystal display panel having a relatively wide area, however, it is difficult to efficiently use the light of the light emitting element because of the structure, and a high luminance light emitting element or a plurality of light emitting elements are required, and there is a tendency that the cost becomes high.
On the other hand, for example, as disclosed in Japanese Patent Unexamined Publication No. 2000-47175 or No. Hei. 10-39778, a just under type of illumination system is also widely adopted in which a liquid crystal display panel is illuminated by a light source disposed just under the liquid crystal display panel. This type of backlight device is superior in that light from the light emitting element can be efficiently used, however, especially in the case where a dot-like light emitting element is used, uneven illumination is apt to occur, and in order to obtain uniform plane emission, it is necessary to secure a distance between the light emitting element and the liquid crystal display panel to a certain degree, and it is not suitable for reduction in size.
Under such circumstances, in the backlight device using the just under type of illumination system, for example, one or plural translucent sheets each being milk-white and having light diffusibility are often used to make the illumination uniform. However, the adoption of such a translucent sheet causes a drop in luminance, and under the existing circumstances, the number of light emitting elements must be increased. Then, it is conceivable to adopt a prism sheet as means for compensating an insufficiency of luminance due to the translucent sheet. A prism sheet in which its one surface is a plane surface and the other surface opposite to the plane surface is a prism surface, is put on the market, plural rows of minute prism portions each having a substantially V-shaped section are formed on the prism surface, the prism surface is normally faced toward the front side (side of the liquid crystal display panel), and the prism sheet is superposed on the front of the translucent sheet and is used. By using the prism sheet as stated above, the light which passes through the translucent sheet and is diffused, is refracted in such a direction as approaches the vertical direction with respect to the liquid crystal display panel to the utmost, and the illumination luminance is raised.
However, in the structure in which the prism sheet is disposed on the translucent sheet so that the prism portion is faced toward the front side, although the illumination luminance is certainly improved, it does not contribute to the equalization of illumination very much, and a sufficient effect can not be obtained for the resolution of the uneven illumination. Thus, it is necessary to take such measures that for example, mask printing of dots or the like is performed for the translucent sheet, or the number of light emitting elements is increased, however, there are demerits that the mask printing of the dots or the like needs a bright light emitting element and unevenness of luminance is apt to occur, and the increase of the light emitting elements causes an increase in cost.
The present invention has been made in view of the problem, and provides a backlight device which is bright and has low unevenness of luminance.
A backlight device according to the present invention includes a translucent sheet having light diffusibility, a light emitting element disposed behind the translucent sheet, and a prism sheet disposed between the light emitting element and the translucent sheet and having a prism surface on which a plurality of prism portions each having at least a pair of inclined surfaces intersecting with each other at a top are formed, and a plane surface portion formed on a surface opposite to the prism surface, in which the prism sheet is disposed away from the light emitting element and the translucent sheet so that the prism surface is faced toward the light emitting element.
Besides, a backlight device according to the present invention includes a translucent sheet having light diffusibility, a light emitting element disposed behind the translucent sheet, and a prism sheet disposed between the light emitting element and the translucent sheet and having a prism surface on which a plurality of prism portions each having at least a pair of inclined surfaces intersecting with each other at a top are formed, and a plane surface portion formed on a surface opposite to the prism surface, in which the prism sheet includes a first prism sheet and a second prism sheet disposed away from each other, a prism surface of the first prism sheet is faced toward the light emitting element, and a prism surface of the second prism sheet is faced oppositely from the first prism sheet.
Besides, a backlight device according to the present invention includes a light emitting element, a prism sheet disposed in front of the light emitting element and having a prism surface on which a plurality of prism portions each having at least a pair of inclined surfaces intersecting with each other at a top are formed, and a plane surface portion formed on a surface opposite to the prism surface, and a translucent sheet disposed in front of the light emitting element and having light diffusibility, in which the prism sheet includes a first prism sheet and a second prism sheet disposed away from each other, a prism surface of the first prism sheet is faced toward the light emitting element, a prism surface of the second prism sheet is faced oppositely from the first prism sheet, and the translucent sheet is disposed behind the second prism sheet and away from the first prism sheet.
Besides, in the backlight device according to the present invention, top edge lines of the respective prism portions are extended in parallel with each other to form a row shape.
Besides, in the backlight device according to the present invention, top edge lines of those are concentrically extended to form a row shape.
Besides, in the backlight device according to the present invention, top edge lines of the prism portions of the first prism sheet and top edge lines of the prism portions of the second prism sheet are extended in parallel with each other.
Besides, in the backlight device according to the present invention, the light emitting element is made of a dot-like light source.
Besides, in the backlight device according to the present invention, the light emitting element is made of a light emitting diode.